The treatment of industrial liquid effluents contaminated by heavy metals has attracted intense interest in recent years, especially for nuclear waste remediation. Disposal of radioactive waste is horrendously expensive. As environmental, political and public health entities place more focus on Zero Liquid Discharge strategies, nuclear industry is now required to treat or eliminate waste streams to a much higher standard than ever before. An urgent need has arisen for new technologies to remove radionuclides such as highly toxic Cs137 producing only solid waste for disposal. The selective removal of Cs significantly reduces the toxicity of the residual waste and offers an opportunity of reutilization of radiocesium as a source of gamma radiation for several purposes both industrial and medical, for example to treat certain types of cancer or for industrial measurement gauges, including moisture, density, leveling, and thickness gauges etc.
Separations-technology development of nuclear wastes is ongoing for a half of century. Three main categories of methods are currently applied: liquid-liquid extraction, sorption and coprecipitation processes.
Sorption, also called ion-exchange chromatography is typically used for dilute solutions, to collect and concentrate species, when the use of organic solvents is not desired, and when the column media may be part of a final waste form (incorporation in a borosilicate glass). However, ion-exchange materials such as ammonium molybdophosphate and crystalline silicotitanate cannot sorb radionuclides such as Cs from highly concentrated and alkaline raw wastes. In addition, they generally exhibit a poor selectivity.
Coprecipitation processes consist in precipitating the radionuclide with a precipitant. As an example, cesium may be removed from liquid waste by coprecipitating it with hexacyanoferrates (HCFs) of divalent transition or heavy metal cations, such as ZnHCF[1] or NiHCF[2], and subsequent flotation of the precipitate. However, this method leads to quite large volumes of secondary wastes, namely inorganic sludges containing a high content of water. In addition, theses sludges can hardly be treated as they are chemically unstable.
Liquid-liquid extraction processes of radionuclide are typically used when a radionuclide(s) is to be separated from solutions with large concentrations of other metals. This method consist in contacting at countercurrent an aqueous waste solution with an immiscible organic solvent containing a complexing agent as extractant, such as tributylphosphate, tripyridyltriazine, bistriazinylpyridines (GANEX, PUREX, DIAMEX and SANEX processes)[3], [4], or calixarene crown ether (U.S. Pat. No. 6,174,503). However, in spite of its good selectivity, this method generally requires the use of organic solvents, acidification of the waste, which reveal costly. Further, the stripping procedure required to regenerate the immiscible organic solvent containing the loaded extractant leads to quite large volumes of liquid wastes (secondary liquid wastes), which may need to be further concentrated. In addition, extractant is generally progressively lost into the aqueous phases during the successive runs, thus further increasing the cost of liquid-liquid extraction processes.
A solid-liquid extraction method for removing cesium from model nuclear waste has also been reported (A. Duhart, et al., Journal of Membrane Science 185 (2001) 145-155). This method involves a solid membrane composed of an unsymmetrical calix[4]arenebiscrown-6 bonded to an immobilized polysiloxane backbone. However, the extraction efficiency of this membrane is much lower than that of liquid-liquid extraction process implementing the same calixarene and/or requires higher amounts of calixarene grafted in the solid phase. Further, the cesium/sodium selectivity is very low as compared to that of liquid-liquid extraction process.
Thus, there is a need for a method of treatment of radionuclides contaminated aqueous and/or organic solutions that overcome the drawbacks of the methods of the state of the art.